UT Technique |  Petrochemical Plants | Boilers | References

DEVELOPMENT OF NDE TECHNOLOGY
Anmol Birring, President of NDE Associates, Inc., has been involved with the development of NDT techniques for detection of HTHA since 1981. Most of the inspections at that time used the ultrasonic amplitude technique, the reliability of which was always questionable. Mr. Birring worked on several projects on this subject with JGC, Japan, Idemitsu, Japan, EPRI and Chevron. During that period, he identified the relationship of ultrasonic backscatter with HTHA. In 1990, he was awarded US Patent, 4,890,496 for developing this technology. Since then backscatter technique has been widely used worldwide in the petrochemical plants.

We provide inspection services worldwide for HTHA assessment.
 

Hydrogen attack (a) The dark area on the Boiler Tube ID represents hydrogen damage. (b) Hydrogen Attack in the 18 mm thick pipe sample from a failure in a Refinery de-suphurization plant in Japan. Failure date March 31, 1982. Note the depth of HTHA at failure is 70 percent.

RELIABILITY OF HTHA INSPECTION
The reliability of HTHA inspections depends on the skill of the inspector. We have run into cases where inspectors missed last stages of HTHA. Plant owners must carefully select inspectors to perform these inspections.
One question to ask would be if the inspector has ever found HTHA in previous inspections ? There are inspectors who have inspected over hundred components but never reported HTHA

SCHEDULING EQUIPMENT FOR HTHA INSPECTIONS

1. C-1/2 Mo Equipment operating above the API 941 limits must be replaced. Piping must be replaced immediately because of its low thickness. 
2. Reactors, heat exchangers shells and other thicker equipment must be inspected at the earliest if immediate replacement is not possible. Ultrasonic backscatter inspection must be performed by skilled inspectors who have a track record of detecting actual HTHA in the plant equipment, not just inspecting for HTHA.
3. Selection of inspection locations is critical. Inspection must be performed at the hottest locations of the equipment. HTHA susceptibility is highly dependent on the heat of the plate and is not very well understood. Just a small difference in heat input or treatment can make a major difference in HTHA susceptibility. So one plate/spool may be totally free of HTHA while the adjoining plate/spool operating at the same temp/pressure conditions in the same vessel may have high levels of HTHA. Each and every plate of the vessel must therefore be tested at both ends. Preferred locations for HTHA are the intersection points of the plates/spool.
4. Any suspect areas detected by ultrasonic testing must be verified by replication of magnetic particle testing from the inside surface.
5. After the inspection, equipment operating above the API 941 limits must be planned for replacement. In the meantime, regular inspections must monitor for any HTHA.
6. HTHA is dangerous. Do not take any chances. Most NDT inspectors performing HTHA inspections miss HTHA. We have run into cases where skilled inspectors have missed last stages of HTHA in the base metal. HTHA in the weld HAZ is difficult to detect and can be easily missed.

REACTOR TAKEN OUT OF SERVICE in france

A case in point is inspection of a C-0.5 Mo reactor that was tested by NDE Associates, inc. in 2004. The reactor at the Total refinery in Le Harve, France was placed in service in 1972 . The refinery was concerned of any damage by HTHA as the reactor was operating above the C-steel curve. In 2004 NDE Associates inspected the reactor using the ultrasonic backscatter technique and found the depth to be about 40 percent of the thickness, indicating significant levels of HTHA. Follow-up replication confirmed HTHA. Further metallurgical and magnetic particle tests from the reactor ID showed severe HTHA.  Based on the results, the refinery decided to take reactor out of service. The experience showed the effectiveness of the inspection in  removing the equipment out service and avoiding a failure. (Note: Prior to this inspection another company in 2002 missed HTHA)

NDE Associates, Inc. has performed several HTHA inspections.  Some of  our clients are:

• Total Fina, Le Harve, France, 2004
• Total, Rome, Italy, 2005
• Total, Milford Haven, UK, 2007
• Chevron Oil, Richmond Refinery, CA.  Conduct follow up inspections after the April 1989 fire caused by Hydrogen Attack failure.
• Chevron, Pascagoula, MS
• BP, Texas City, 2005
• other refineries in US, Canada
• Huntsman Chemicals, Melbourne, Australia.
• SK Oil, Korea. Conduct follow up inspections after the failure caused by Hydrogen Attack. Failure date: May 13, 1999.
• CF (Fertilizers) Industries, Donaldsonville, Louisiana.
• Conoco Oil Company, Billings Refinery, MT and Denver Refinery, CO.

Background

Steel suffers from hydrogen damage or high temperature hydrogen attack when a seepage of hydrogen reacts with metal carbides to form methane gas. This reaction decarburizes the steel, produces micro cracks, and lowers the toughness of the steel, but does not necessarily cause a loss of thickness. Detection of hydrogen attack is important to ensure the safe operation of pressure vessels and piping that is susceptible to such damage.

MC + 4H = M (M:metal) +CH₄ (methane)

The type of damage caused by hydrogen attack depends on the source. The source of hydrogen in boiler tubes is from a reaction of steam and steel. Hydrogen damage in boiler tubes is, therefore, always associated with ID corrosion. However, in chemical plant the source of the hydrogen is from the flow stream (hydrocarbons) and, therefore, there is no corrosion associated with hydrogen damage.

Damage caused by HTHA failure in a refinery

Petrochemical and Ammonia Plants

High temperature hydrogen attack (HTHA) in the petrochemical and ammonia plants is caused when the hydrogen from the stream seeps into carbon and low alloy steels at high temperatures. Hydrogen reacts with the carbides in the steel, decarburizing the steel and forming methane gas bubbles at the grain boundaries with no loss of thickness. The methane gas bubbles grow with time and result in micro cracking. The combination of micro cracking and decaburization reduces the fracture toughness of steels and lead to major failures.

HTHA is triggered in the components operating at high temperatures and high hydrogen partial pressure. API 941's Nelson curves provide information about the safe operating environment for components operating in hydrogen environments (1). Components operating in the unsafe environments are susceptible to hydrogen attack. The concern is especially true for C-1/2 Mo steels whose curve has been been lowered and now dropped to the same level as that carbon steel (1). Because of this drop, some of the C-1/2 Mo components originally operating in the safe region are now in the unsafe region of the Nelson curves. These components should either be inspected regularly for evidence of HTHA or replaced with a higher grade of steel. 

Hydrogen Attack can occur both in the base metal and the weld HAZ. The attack in the base metal is wide spread and distributed uniformly. In addition to temperature and hydrogen partial pressure, the susceptibility of attack also also depends on the microstrocture ferrite/bainite or ferrite/pearlite with quasi M₂₃C₆ carbide (2). Weld attack is very localized and grows along the HAZ. The susceptibility of attack in the weld HAZ depends on the heat treatment. Post-weld heat treated welds are less susceptible to hydrogen attack compared to the welds that are not heat treated. There have been several cases where the depth of the attack in the HAZ is greater than the depth in the base metal. It is therefore imperative that both the base metal and the weld HAZ be inspected for presence of hydrogen attack.

 Some of the factors that are important for scheduling inspections are as follows:

• operating conditions (partial pressure of hydrogen and temperature) relative to the operating limits provided in API 941, 1997 edition (1).
• slow cooled C-1/2 Mo steels have less resistance to hydrogen attack than normalized steels (2).
• post-weld heat treated welds are less susceptible to hydrogen attack compared to the welds that are not heat treated.

A general discussion of HTHA prediction is given in reference 5.

NDT Techniques for Petrochemical and Ammonia Plants

Base metal attack is detected by using a combination of ultrasonic back-scatter and velocity measurements (6,7, 8). Hydrogen attack increases the ultrasonic backscatter and reduces the ultrasonic velocity in the material. In particular, HTHA increases the ratio of S-wave to L-wave velocities. The backscatter and velocity ratio measurements are applied to detect hydrogen attack. The ultrasonic backscatter technique was developed by A. S. Birring in 1989 and was first applied at the Chevron Richmond Refinery in 1989 (7).  Details of the UT techniques for HTHA inspection are given in references 6 and 7. 

Application of the ultrasonic techniques for HTHA detection requires an individual with a good understanding of the mechanism of HTHA and how it affects the propagation and scattering of ultrasonic waves.  One has to understand that that while hydrogen attack affects velocity-ratio, backscatter and the frequency of the reflected signal, other material anomalies can  influence these ultrasonic parameters as well and give a false call. Ultrasonic  inspection for this application is therefore not straight forward and requires a logical test methodology to detect HTHA.

Phased Arrays are not recommended for detection of base metal HTHA

Application of ultrasonic backscatter for detection of HTHA. The ultrasonic backscatter is ID connected.

Weld HAZ attack is detected using the ultrasonic shear wave technique. Since the cracking caused by hydrogen attack is in the weld HAZ is extremely fine, the shear wave inspection is done at a very high sensitivity.

Both the base metal and weld HAZ should be inspected for hydrogen attack.

Further verification of attack can be done by surface replication.

Note: This website is solely for informational purposes. In no event will this company be responsible for any loss or damages resulting from any viewer's use of these materials.

References

1. Steels for Hydrogen Service at Elevated Temperatures and Pressure in Petroleum Refineries and Petrochemical Plants, API Recommended Practice 941, Fifth edition, American Petroleum Institute, 1997.
2. T. Ishiguro, H.Yamamoto, K. Kawano, et al, "Metallurgical Effect on Hydrogen Attack Damage in C-½Mo Steels," Proceedings, 1996 ASME/ICPVT Pressure Vessels and piping Conference, 21-26 July, 1996
3. Hattori, K. and Aikawa, S., "Scheduling and Planning Inspections of C-0.5Mo Equipment Using the New Hydrogen Attack Tendency Chart," PVP vol 239/MPC-vol 33, Serviceability of Petroleum Process and Power Equipment, ASME, 1992.
4. K. Kawano, "Recent Activities in High Temperature Hydrogen Attack," to be presented in 2005.
5. G. R. Prescott, "History and basis of Prediction of Hydrogen Attack of C-1/2 Mo Steel," Material Property Conference, Vienna, Oct 19-21, 1994.
6. A. S. Birring, et al. "Method and Means for Detection of Hydrogen Attack by Ultrasonic Wave Velocity Measurements" US Patent, 4,890,496, January 2, 1990
7. A. S. Birring and K. Kawano, "Ultrasonic Detection of Hydrogen Attack in Steels," Corrosion, March, 1989.
8. A. S. Birring, M. Riethmuller, and K. Kawano, "Ultrasonic Techniques for Detection of High Temperature Hydrogen Attack," Materials Evaluation, February, 2005.

Highlights

NDE Associates inspections detect HTHA in a reactor.

HTHA inspection of a reactor in France.
The reactor was taken out of service after inspections found HTHA.

Microcracks and decarburization by HTHA from the failure in Japan March 31, 1982.

NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944    Fax: 281-488-8485